Implantable cardiac pacemakers are a class of cardiac rhythm management devices that provide electrical stimulation in the form of pacing pulses to selected chambers of the heart. (As the term is used herein, a pacemaker is any cardiac rhythm management device with a pacing functionality regardless of any additional functions it may perform such as cardioversion/defibrillation.) Pacemakers typically have a programmable electronic controller that causes the pacing pulses to be output in response to lapsed time intervals and sensed electrical activity (i.e., intrinsic heart beats). Most pacemakers are programmed to operate in a so-called demand mode (a.k.a., synchronous mode), where a pacing pulse is delivered to a heart chamber during a cardiac cycle only when no intrinsic beat by the chamber is detected. An escape interval is defined for each paced chamber, which is the minimum time interval in which a beat must be detected before a pace will be delivered. The ventricular escape interval thus defines the minimum rate at which the pacemaker will allow the heart to beat, sometimes referred to as the lower rate limit. If functioning properly, the pacemaker in this manner makes up for a heart's inability to pace itself at an appropriate rhythm.
In order for a pacemaker to control the heart rate in the manner described above, the paces delivered by the device must achieve “capture,” which refers to causing sufficient depolarization of the myocardium that a propagating wave of excitation and contraction result (i.e., a heart beat). A pacing pulse that does not capture the heart is thus an ineffective pulse. This not only wastes energy from the limited energy resources (battery) of pacemaker, but can have deleterious physiological effects as well, since a demand pacemaker that is not achieving capture is not performing its function in enforcing a minimum heart rate. A number of factors can determine whether a given pacing pulse will achieve capture, but the principal factor of concern here is the energy of the pulse, which is a function of the pulse's amplitude and duration. The minimum pacing pulse energy necessary to achieve capture by a particular pacing channel is referred to as the capture threshold. Programmable pacemakers enable the amplitude and pulse width of pacing pulses to be adjusted, along with other parameters. It is common practice to determine the capture threshold by initially pacing with a high energy to ensure capture and then progressively lowering the pacing pulse energy during a sequence of cardiac cycles until capture is no longer achieved. The pacing pulse energy can then be adjusted to an appropriate value in accordance with the determined capture threshold by setting it equal to the capture threshold plus a specified safety margin.
A common technique used to determine if capture is present during a given cardiac cycle is to look for an “evoked response” immediately following a pacing pulse. The evoked response is the wave of depolarization that results from the pacing pulse and evidences that the paced chamber has responded appropriately and contracted. By detecting an evoked atrial or ventricular depolarization that exceeds a specified value (i.e., corresponding to an evoked P-wave or evoked R-wave, respectively, on a surface electrocardiogram or their equivalents in an internal electrogram), the pacemaker is able to detect whether the pacing pulse (A-pulse or V-pulse) was effective in capturing the heart, that is, causing a contraction in the respective heart chamber. Capture verification can be performed in the clinical setting, with the clinician then adjusting pacing parameters so that the heart is reliably paced. It is desirable, however, for the pacemaker itself to be capable of verifying capture so that loss of capture can be detected when it occurs with pacing parameters then adjusted automatically, a function known as autocapture. (See, e.g., U.S. Pat. No. 6,169,921 issued to KenKnight, et. al. and presently assigned to the Guidant Corp.) An autocapture function provides the pacemaker with extended longevity, greater ease of use, and greater patient safety.
Also included within the concept of cardiac rhythm is the manner and degree to which the heart chambers contract during a cardiac cycle to result in the efficient pumping of blood. For example, the heart pumps more effectively when the chambers contract in a coordinated manner. The heart has specialized conduction pathways in both the atria and the ventricles that enable the rapid conduction of excitation (i.e., depolarization) throughout the myocardium. These pathways conduct excitatory impulses in a manner that results in a coordinated contraction of both atria and both ventricles. Without the synchronization afforded by the normally functioning specialized conduction pathways, the heart's pumping efficiency is greatly diminished. Patients who exhibit pathology of these conduction pathways, such as bundle branch blocks, can thus suffer compromised cardiac output. The resulting diminishment in cardiac output may be significant in a patient with congestive heart failure (CHF) whose cardiac output is already compromised. Intraventricular and/or interventricular conduction defects can also be a cause of CHF in some patients. In order to treat these problems, pacemakers have been developed which provide multi-site electrical pacing stimulation to one or both of the atria and/or ventricles during a cardiac cycle in an attempt to improve the coordination of atrial and/or ventricular contractions, termed cardiac resynchronization therapy. To optimize the cardiac output for some heart failure patients, for example, the right and left ventricles are paced synchronously with a determined time offset, termed biventricular pacing.
Multi-site resynchronization pacing, however, is problematic for conventional capture verification methods based upon evoked response detection as described above. In biventricular pacing, for example, the proximity in time of resynchronization paces to the left and right ventricles may prevent an evoked response caused by the first pace from being distinguished from the second pace. In addition, the second pace could interfere with evoked response sensing when the evoked response from the first pace occurs within an amplifier blanking interval initiated by the second pace.